Pneumatic tire low inflation warning systems have been proposed for decades. However, little commercialization of these systems has occurred for any one of a number of reasons, such as: high cost; low reliability; difficulty of installation; and low consumer demand. Now, however, with increasing emphasis on increasing safety, reducing automobile size and weight, automotive safety, elimination of the spare tire and use of puncture-resistant self-sealing tubeless tires, automobile manufacturers are renewing their interest in these systems. The criteria that must be met are many: (1) low cost; (2) uncomplicated assembly and installation; (3) suitability for use with self-sealing tires; (4) operatibility over the expected life of the automobile; and (5) capability of functioning to generate warning signals when a tire is (a) completely flat, (b) "nearly flat" and (c) "too low", (6) failure mode characteristics.
Category (4) above is particularly important. Using a modern, radial automobile tire as an example, most drivers cannot detect a low pressure condition even when a tire pressure as low as 5-9 psig exists. A tire pressure that is lower than normal should be detected even if not readily apparent. For example a radial tire with that pressure range could be characterized as "nearly flat". Moreover, when a radial tire has a pressure below about 15 psig, prolonged use could significantly reduce its life span. Thus, a radial tire pressure below about 15 psig could be characterized as "too low". When a tire is completely flat, although one would expect all drivers to be cognizant of that condition, the warning device must function and must also survive for continued use when the flat tire is replaced or repaired and reinstalled. Finally the device should not damage the tire when driven on in unactivated mode.